How to map an image on a texture?
Tab into edit mode, and in top mode, press U -> Project From View (Bounds).



This should unwrap your plane properly so that the image maps across it correctly.



If your image isn't square, you'll need to do some extra scaling (of the plane) in the UV Image Editor to get it to match correctly, or you'll get stretching on the plane.

Then just apply the texture in the material.